A method of measuring chronological dielectric breakdown characteristics of an insulating film, as in constant voltage time-dependent dielectric breakdown (TDDB) measurement, constant current TDDB measurement and the like, is commonly employed for evaluating an insulating film used in a semiconductor device.
The constant voltage TDDB measurement is an evaluation method that utilizes a time-dependent breakdown phenomenon in which a constant voltage not larger than the breakdown voltage is applied to the insulating film long enough to cause the dielectric breakdown of the insulating film dependent to a voltage application time. Constant current TDDB measurement is an evaluation method that utilizes a time-dependent dielectric breakdown phenomenon in which a constant amount of current is applied to the insulating film long enough to cause the dielectric breakdown of the insulating film dependent to a current application time.
FIG. 5 shows a diagram illustrating a measurement circuit for an insulating film evaluation through a constant current application. A semiconductor substrate 1 of one conductivity type includes a first diffusion layer 2 of the opposite conductivity type. An insulating film 3 and an electrode 4 are laminated on the first diffusion layer 2 to form a MOS diode. The electrode 4 of the MOS diode is connected to a terminal 5a and the diffusion layer 2 is connected to a terminal 5b. The terminal 5a and the terminal 5b are wired to a current source 8 via a switch 6a and a switch 6b, respectively, to apply current between the terminals 5a and 5b. A voltmeter 7 is connected in parallel to the current source 8. In this measurement circuit, a current is applied to the insulating film in one direction, and the direction of the current is chosen to be forward or backward to apply for the measurement of dielectric breakdown characteristics (see JP 06-201761 A, for example).
However, both the forward direction current and the backward direction current should be applied to the MOS diode in order to guarantee the endurance cycling of a semiconductor memory device. To apply both the currents in a conventional measurement circuit, the forward direction current and the backward direction current have to be applied alternately, taking a long period of time. Another problem is that, because a current is applied in both directions to the same insulating film, the evaluation result varies depending on the length of the current application time.